The present invention relates to an Auxiliary Power Unit (“APU”) for an aircraft with reduced weight, but which at the same time meets all fire-related airworthiness standards. More particularly, the invention refers to an APU with integrated sensing means for detecting temperature under fire conditions.
Another object of this invention is to provide an APU that complies with the standards and safety requirements for APUs and their housings, especially for maintaining them available and securely attached to the aircraft body in the event of a fire.
Another object of this invention is to provide an APU system with faster response against fire, when a fire occurs in the APU compartment.
An APU in an aircraft is a small turbine engine designed to provide electrical, hydraulic or pneumatic power with compressed air, mainly when the main engines of the aircraft are not running. The APU provides electrical power and pressurized air for the pneumatic systems and typically feeds the air conditioning system, the main engine starting system and the anti-ice system of an aircraft.
Because it can be an essential and an auxiliary component, the APU must meet stringent airworthiness standards that ensure that the APU is safe in any condition. For this purpose, airworthiness regulations set the specifications needed for complying with the certification requirements in order to satisfy a safe APU, those requirements under fire conditions among others.
Traditionally, the certification requirement specified a fireproof APU. Special attention is paid in critical areas considered as the most likely areas to start up or suffer a fire, such as the combustion chamber of the APU or areas where flammable fluid could be accumulated. So, in addition both APU and its compartment were conventionally surrounded by a detector wiring which routed around the critical areas of the APU and its compartment for detecting temperature, acting as a fire detection system. Typically, the detecting wire detects temperature increase through pressure variation and is lead to an electrical signal transducer, which feeds a fire detection unit for triggering a fire detection alarm when the pressure variation exceeds a pre-established threshold. For illustrative purpose, FIG. 1 shows the detector wiring 4 routing along the APU compartment 8. In addition the APU machine must satisfy stringent requirements for its relevant qualification. This qualification obliges to demonstrate that the APU is safe under a fire condition. This must be demonstrated either by construction or by protection. If chosen the first option, the manufacturer must demonstrate typically by testing on many parts with a standardized flame and time duration.
APU parts and components are designed according to the temperature that they will have to withstand in normal operating conditions. Therefore, the combustion chamber of the APU is in most part or in its entirety made of steel or similar material that withstand very high temperatures, which is fireproof of itself, whereas, for instance, the gearbox housing of the APU is made of aluminum.
Since the front part of the APU, particularly the gearbox housing, typically includes mounts through which the APU is attached to the APU compartment, the integrity of said mounts has to be ensured in order to avoid the loss of the APU attachment to its compartment in the event of a fire.
To solve this problem, there are known solutions that comply with the fireproof certification requirement for the entire APU based on structural reinforcing solutions.
Complying with the requirement entails more difficulty when the gearbox housing is made of aluminum or its alloys, since the risk of failure increases. One solution involves using a stronger material for the housing, thereby, the gearbox housing will be able to withstand a fire for a longer time. However, this solution implies an undesirable increase of weight and might entail a gearbox housing re-designed, which often used aluminum alloy to reduce the overall weight of the APU.
Other solutions address the weak configuration problem with the inclusion of ancillary pieces to the APU. Usually, these ancillary pieces are made of steel, titanium or fireproof-type materials and consist of brackets or rings. These pieces strengthen the support of the APU to the aircraft and carry loads from the gearbox. Nevertheless, although this type of solutions results in a reinforced APU assembly allowing APU to better withstand fire conditions, the solution comes with the drawback of the weight addition.